Prepolymers bearing isocyanate groups are commonly used in industry to make a wide variety of polyurethane products. These prepolymers are usually prepared by reacting a polyol with an excess of a monomeric polyisocyanate, for example, a diisocyanate such as methylene diphenyl diisocyanate (MDI). Disadvantages associated with the process for forming these prepolymers, involve the unreacted monomers.
In the art, several methods exist to reduce free monomer content in prepolymers. Monomers can be stripped by distillation, more in particular for higher vapor pressure diisocyanate based prepolymers. Methylene diphenyl diisocyanates, on the other hand, have very low vapor pressures. Furthermore, vapor pressure is proportional to concentration. This combination makes it extremely difficult to remove methylene diphenyl diisocyanate monomers from prepolymers down to extremely low levels. In addition, the temperature during distillation is preferably not raised above 220° C., since MDI may start to form carbodiimides and CO2. This process is expensive, requires high cost equipment, and a high investment for low vacuum pumps.
Alternatively, free monomer content can be reduced by asymmetric prepolymerization. However, as a result, the reactivity and curing behavior with water can be extremely low due to the full 2,4-isomer content. The mechanical properties for such prepolymers are less desirable as well. Additionally, raw material availability and price may be an issue.
Therefore, there remains a need for prepolymers and processes to prepare said prepolymers that overcome one or more of the aforementioned issues. It is an object of the present invention to overcome one or more of the aforementioned issues. More in particular, it is an object of the present invention to reduce free monomer content in prepolymers. More in particular, it is an object of the present invention to reduce free monomer content in prepolymers, while keeping good processability thanks to low viscosity. More in particular, it is an object of the present invention to reduce free monomer content in prepolymers, while keeping good mechanical properties. More in particular, it is an object of the present invention to reduce free monomer content in prepolymers, while keeping good processability thanks to low viscosity and good mechanical properties.